The applications for antennas continue to increase as antenna sizes are reduced and complimentary broadband microwave designs are developed. In this regard, the evolution of thin-film "patch", or microstrip, antennas has been particularly important. Chapter 7 of R. Johnson & H. Jasik, Antenna Engineering Handbook (2d ed. 1984) provides an excellent discussion of such antennas.
In the production of thin-film patch antennas, a dielectric substrate is typically coated on both sides with a thin film of metal (i.e., less than 0.5 mil), or alternatively, a thin metal foil is laminated to the opposing sides of the substrate. Using conventional photolithographic/ etching techniques, the metal on one side is then selectively removed to yield a high-resolution patch of a desired configuration. The metal on the other size serves as a ground plane for microwave transmission/reception.
In order to satisfy broadband and other signal requirements for many expanding applications, it is essential for thin-film patch antenna substrates to comply with extremely tight flatness, thickness and dielectric range tolerances and/or to implement extensive tuning networks. This is due, in large part, to the fact that the thin metal patch cannot readily be adapted, or tuned, to compensate for substrate deviations. To achieve flatness, thickness and dielectric constant range requirements, substrate production processes must be tailored and closely controlled, and substrate preconditioning (i.e., grinding) may be necessary. As will be appreciated by those skilled in the art, such demands, coupled with the attendant labor/equipment demands of photoetching techniques, render thin-film patch antennas impractical from a cost standpoint for many potential antenna applications.
For example, to realize the full potential of Global Positioning Systems (GPS), the need for low-cost receivers for truck fleets, surveying and navigation equipment, etc. is particularly acute. While high-resolution patches can be configured by the noted thin-film production techniques to meet the broadband, low-angle gain needs of GPS receivers for L.sub.1 and L.sub.2 band operations (centered on approximately 1.575 GHz and 1.227 GHz, respectively), attendant costs preclude widespread application. Cost considerations are further compounded when ceramic substrates are considered. That is, while ceramic substrates can provide high dielectric constants (e.g., as high as 9 to 10), thereby permitting antenna size reduction, the costs associated with satisfying substrate flatness, thickness and dielectric constant tolerances become prohibitive. In view of the foregoing, thin-film patch antennas have been unable to meet the needs of many potential applications and have failed to capitalize on ceramic-related benefits for GPS or other similar applications.